"When floods cover floodplain vegetation for several days, it starts to die and rot, and this process strips oxygen from the surrounding water.

We are expecting the trend of decreasing oxygen levels will continue over the coming days, resulting in fish gasping at the surface, or fish deaths.

"This 'dead' or 'black water' then flows back into the river via the flood plain drainage network.

"Warmer water temperatures also exacerbate the situation, speeding up decomposition processes.

"Other factors that contribute to the deoxygenation of river water during summer floods include heavy sediment load in the river, decaying in-stream organic matter, mobilisation of drain mud and chemical processes associated with the drainage of acid sulphate soils.

"Summer floods and low oxygen levels were a major factor in fish kills on the Richmond and Clarence rivers in the summers of 2001 and 2008."

Mr Riches said DPI is receiving reports from councils on the Richmond River that dissolved oxygen levels are starting to fall and water temperatures are increasing.

"This is concerning as water covering major coastal floodplains have yet to fully drain into the main river systems and warm, shallow flood waters low in oxygen are yet to drain back into the river," he said.

"We are expecting the trend of decreasing oxygen levels will continue over the coming days, resulting in fish gasping at the surface, or fish deaths.

"The extent and scale of fish deaths is very difficult to determine as fish can sometimes move ahead of this poor water, while at other times they are not able to get away and this can result in significant fish kills.

"Councils and the Northern Rivers Catchment Management Authority have been undertaking rehabilitation works on north coast floodplains to try and reduce the scale of these events over the past decade.

"However, the size of our floodplains means that there is still more work to be done to further reduce the risk of large fish kills after flood events in these rivers," Mr Riches said.